1. Field of the Invention
The present invention relates to a method and a device for unwinding a sawing wire from a reel or winding a sawing wire onto a reel, in which the reel rotates and the sawing wire runs from the reel or toward the reel via a pulley.
2. The Prior Art
Devices of this type are used in wire saws, in particular in wire saws for cutting wafers of semiconductor crystals. These wire saws have a cutting head with two or more wire-guide rollers, around which a sawing wire is wound. Between adjacent wire-guide rollers, the sawing wire forms a wire web, which in conjunction with a sawing suspension can be used as a cutting tool. If a sawing wire is used which is coated with a bonded abrasive grain, then no sawing suspension is needed.
During the cutting operation, the sawing wire runs from a stock reel, via a system of pulleys, to the wire-guide rollers, and from there, via a system of pulleys, back to a take up reel. The stock reel and the take up reel have to be dimensioned in such a way that they are able to hold a sawing wire with a length of several hundred kilometers. Such enormous wire lengths are necessary in order to divide large-diameter crystals into wafers. In the take-up state, the reels have a correspondingly high weight.
As a rule, the length of the sawing wire employed needs to be increased, wherever the diameter of the crystal to be cut is increased. It is sometimes necessary to cut crystals of particularly large diameters, for example silicon single crystals with diameters of 200 mm and greater. In this situation, the wire saw can be operated with an oscillating direction of rotation of the wire-guide rollers (oscillating operation). In this type of operation, some of the sawing wire which has previously been unwound from the stock reel is wound back onto the stock reel. This occurs after the direction of rotation of the wire-guide rollers has been reversed, and this operation is repeated periodically. In this way it is possible to operate for a longer period using the stock of wire available than would be possible when using the wire saw with a constant direction of rotation. It is advantageous if the reel body of the stock and take up reel is designed to be as long as possible and to have a diameter which is as small as possible. The moments of inertia which arise during the change in the direction of rotation are lower than with reels that are shorter and have larger diameters.
In the prior art, it is the travelling pulley which is moved toward the take up reel parallel to the reel axis in accordance with a predetermined program. This ensures that the wire windings are arranged along the reel axis in the order in which these wire windings are produced. When unwinding from the take up reel during the oscillating operation, the travelling pulley is moved in precisely the reverse sequence.
When winding the sawing wire onto the stock reel during the oscillating operation, the sawing wire windings produced are distributed on the reel in accordance with a predetermined program. This is carried out with the aid of the travelling pulley over which the sawing wire runs before it is wound onto the stock reel. This travelling pulley is moved parallel to the reel axis in accordance with the program specification.
A drawback of this unwinding and winding-up method is that the sawing wire is deflected laterally by the movement of the travelling pulley. This is because the other rollers of the pulley system do not follow the movement of the travelling pulley. The deflection of the sawing wire causes accelerated wear to the affected pulleys. This phenomenon is particularly accentuated when reels are used with an elongated reel body. Moreover, in this method it is necessary to regulate the rotational speed of the reel in a complex manner in order to achieve a uniform pretensioning of the sawing wire. When the travelling pulley is moved parallel to the reel axis, the length of wire between this travelling pulley and the immobile pulleys changes constantly. Hence there is a change in the pretensioning of the sawing wire.
There is also a further drawback in connection with unwinding or winding the sawing wire from or onto a reel. The associated reel mounting and removal were previously carried out largely by hand, and were complex due to the heavy weight of wound reels.